Monitoring of water uptake in organic coatings under cyclic wet–dry condition

Abstract

The absorption of water in the double-layer coatings, comprising epoxy and urethane layers, was monitored under a cyclic wet–dry condition using AC impedance method. Four types of double-layer coatings were prepared according to coating order, namely, epoxy–epoxy (EE), urethane–epoxy (UE), epoxy–urethane (EU), and urethane–urethane (UU) double-layer coatings. The coated specimens were exposed to the alternate conditions of 4 h immersion in a 0.5 M NaCl solution and 8 h drying at 25 °C and 60% RH. The capacitances of coatings were monitored during exposure and the amounts of water absorbed in coatings were estimated from the obtained capacitance. The mechanism of corrosion beneath organic coatings under cyclic wet–dry exposure in chloride-containing environments was discussed on the monitoring results of water uptake.